1. Field of the Invention
The invention relates to compacts based on pyrogenically produced silicon dioxide, a method of producing them and use thereof as catalyst carriers or catalysts.
2. The Prior Art
Pyrogenically produced silicon dioxide is characterized by extremely fine division and a consequently high specific surface area, very high purity, spherical shape of particles and absence of pores. As a result of these properties, pyrogenically produced oxides are becoming increasingly important as carriers for catalysts (D. Koth, H. Ferch, Chem. Ing. Techn. 52, 628 (1980).
Since pyrogenically produced oxides are very finely-divided, there is some difficulty in shaping them to form catalyst carriers or catalysts.
DE-A 31 32 674 discloses a method of producing compacts from pyrogenically produced oxides, using silica sol as the binder.
DE-A 34 06 185 discloses a method of producing compacts using glaze frit powder as a binder and glycerol as a mold release agent.
DE-B 21 00 778 discloses use of granulates based on pyrogenically produced silicon dioxide for production of catalyst carriers in the form of vinyl acetate monomers.
DE-A 39 12 504 discloses a method of producing compacts wherein aluminum stearate, magnesium stearate and/or graphite are used as mold release agents and urea and methyl cellulose are used as pore-forming agents.
These known compacts are commercially available as AEROSIL silicon dioxide pellets number 350, by Degussa A. G. They contain about 0.4 wt. % Mg.
It is known from EP-B 0 519 435 to compress SiO.sub.2 into carriers, using binders, to anneal the resulting carriers and to wash the annealed carrier particles in acid until no further binder actions are given off.
The known methods have the disadvantage that in some catalytic reactions such as vinyl acetate production from ethylene, acetic acid and oxygen, or hydration of ethylene to ethanol, the compacts obtained do not have the required optimum properties such as high purity, high activity, high selectivity, high yield of product and high stability.